Silica is one of major fouling problems in many processes using water. Silica is difficult to deal with because it can assume many low solubility chemical forms depending on the conditions. Below about pH 7 for example, monomeric silica tends to polymerize to form oligomeric or colloidal silica. At higher pH's particularly above about pH 9.5, silica can form monomeric silicate ion. Since conversion can be slow, all of these forms may exist at any one time depending on the history of the system. Furthermore, the silicate ion can react with polyvalent cations like magnesium and calcium commonly present in process waters to produce salts with very limited solubility. Thus it is common for a mixture of many forms to be present: monomeric, oligomeric and colloidal silica; magnesium silicate, calcium silicate and other silicate salts. In describing this complex system, it is common practice to refer to the mixture merely as silica or as silica and silicate. Herein these terms are used interchangeably.
A further complication in controlling silica and silicate fouling is that colloidal silica tends to be more soluble as temperature is raised while polyvalent metal salts of the silicate ion tends to be less soluble with increasing temperature.
Two possible mechanisms for controlling silica or silicate salts from fouling or depositing on a surface during a process are: 1) inhibiting precipitation of the material from the process water, and 2) dispersing the material once it has formed in the bulk water to prevent it from attaching to surfaces. The exact mechanism by which a specific scale inhibitor functions, however, is not well understood. The additives of this invention may be operating by either or both of these routes.
Processes that would likely benefit from a material that could inhibit the deposition of silica or silicate salts from water are, for example: cooling water, boiler water, geothermal process to generate electricity or for heating, and sugar (particularly cane and beet) processing. In each of these processes, heat is transferred to or from the water. In three of these processes, cooling water, boiler water and sugar processing, heat is added to the water and evaporation of some of the water takes place. As the water is evaporated the silica (or silicates) will concentrate. If the silica concentration exceeds its solubility, it can deposit to form either a vitreous coating or an adherent scale that can normally be removed only by laborious mechanical or chemical cleaning. In geothermal processes, hot water laden with silica or silicates is used to heat homes or factories or is converted to steam to drive a turbine and generate electricity. At some point in each of the above four processes, heat is extracted from the water, making any dissolved silicate less soluble and thus likely to deposit on surfaces.
The current practice in each of these four processes is to mechanically limit the amount of silica or silicates that build up in the water so that the catastrophic consequences of deposition of these compounds does not occur. For example in cooling water, the accepted practice is to limit the amount of silica or silicates to about 180 ppm, expressed as SiO.sub.2. In addition, deposition of CaCO.sub.3 (which can act as a nucleating agent for silica or silicates to deposit upon) is controlled by well known inhibitors such as phosphonates or polymers such as polyacrylic acid or polymaleic acid. Reportedly, the current best available polymer for control of silica or silicates in cooling water is polymaleic acid of about 1000-1300 weight average molecular weight. Because the silica is limited to 180 ppm and because in many arid areas of the U.S. and other parts of the world make-up water may contain from 50-90 ppm silica, cooling water can only be concentrated 2 to 3 times before the risk of silica or silicate deposition becomes too great. A polymer that would enable greater re-use or cycling of this silica-limited cooling water would be a great benefit to these areas.
Similarly in boiler water, the American Society of Mechanical Engineers recommends that silica be limited to certain levels depending on the operating pressure of the boiler. For example, in low pressure boilers (less than 300 psig) the amount of silica, as SiO.sub.2, should be kept below 150 ppm. As the pressure is raised, the level of silica that can be tolerated in the recirculating boiler water becomes progressively less. A polymer that would enable boilers to operate at higher cycles of concentration, particularly low pressure boilers where silica volatilization is not a great concern, would allow more energy-efficient use of the heated water.
In sugar production, especially cane sugar where silica levels are highest, the sugar evaporators are cleaned after about 2-3 months to prevent excessive deposition of silica. The cleaning normally involves a vigorous mechanical brushing with harsh chemicals to remove the silica and other salts. An inhibitor that could extend the length of the sugar evaporation processing between cleanings or that would make the cleaning less ardous would extend the life of the evaporators and increase their output during a season.
Geothermal process presently control the temperature drop as a means of preventing the deposition of silica on equipment surfaces. An inhibitor that limits silica deposition would allow the temperature drop in this process to be greater, and allow more efficient use of the heat in the geothermally produced water.
In addition to preventing fouling of surfaces with silica or silicates, an inhibitor or dispersant for this foulant would allow the use of higher levels of silica/silicates for corrosion control. In potable water, silicates are added to the water to prevent "red water" from corrosion of water mains made of ferrous metals. In cooling water, an inhibitor has long been sought after that would enable silica to be used as an a non-toxic corrosion inhibitor.
In enhanced oil recovery, silicates are added to the drive fluid to help move the oil through the formation. An effective silica-inhibitor would prevent the formation from being clogged with metal ion silicates, thus allowing the efficient recovery of the oil from the underground formation.
Other processes that have need of a silica or silicate inhibitor or dispersant are detergent applications such as laundering, cleaning and dishwashing. In laundering, the silica inhibitor would prevent incrusation or stiffening of the fabric by neutral silicates in the water or by silicates added to the detergent formulation as builders. An inhibitor would have the added benefit of preventing deposition of the silica or silicates on surfaces of the washing machine, such as heating elements and plumbing. In dishwashing, silicates are also added as builders and can produce scale and incrustation analogous to laundering.
(Meth)acrylic acid and maleic acid based polymers have long been used in water treatment. Co- and ter-polymers of (meth)acrylic acid with 2-acrylamido-2-methyl propane sulfonic acid (AMPS) in particular have been proposed for inhibiting sulfate, carbonate and phosphate scale as well as for other treatments like removing rust. For example, U.S. Pat. Nos. 3,332,904; 3,692,673; 3,709,815; 3,709,816; 3,928,196; 3,806,367 and 3,898,037 are directed to using AMPS containing polymers. GB No. 2,082,600 proposes an acrylic acid/AMPS/acrylamide polymer and WO No. 83/02607 and WO No. 83/02608 are directed to (meth)acrylic acid/AMPS copolymers as inhibitors of these scales.
In addition U.S. Pat. No. 4,711,725 disclosed the use of terpolymers of (meth)acrylic acid/AMPS/substituted acrylamides for inhibiting the precipitation of calcium phosphate.
The inhibition of silica and silicate scaling specifically has also been addressed in several publications. U.S. Pat. No. 4,029,577 is directed to the use of acrylic acid/hydroxylated lower alkyl acrylate copolymers to control a spectrum of scale imparting precipitates including magnesium and calcium silicates. U.S. Pat. No. 4,499,002 discloses (meth)acrylic/(meth)acrylamide/alkoxylated primary alcohol ester of (meth)acrylic acid for the same purpose. Japanese Patent Disclosures 61-107997 and 61-107998 are directed to polyacrylamide and selected (meth)acrylic acid copolymers to control silica scale.
The term copolymer is widely employed in publications, but not always with the same meaning, sometimes referring to a polymer from only two monomers and other times to a polymer from two or more. To avoid ambiguity, the term copolymer as used herein is defined as a polymer being derived from only two monomers and a terpolymer is a polymer derived from three or more monomers.
Despite the large number of publications in the area of scale inhibitors, none provide an effective method to control the troublesome silica and silicate scale. Limiting the level of silica introduced or allowed to accumulate in the aqueous system is still the primary method of dealing with the problem.
It is, therefore, an objective of this invention to provide a method that effectively inhibits silica depositions in aqueous systems.
It is an objective to provide a chemical method using additives to replace the mechanical techniques of dealing with silica scaling by limiting the concentration of silica allowed to build-up in the system or by the labor intensive removal of silica deposits.